In fork lift trucks, it is common to utilize one or more cylinders to lift the fork of the truck. In such trucks, hydraulic systems usually comprise a directional valve that in one position is in neutral, in a second position provides fluid to the cylinder to lift the forks and a third position permits the fluid to exhaust from the cylinder to lower the forks.
In such a system, it is possible that, upon lowering, the forks engage inadvertently another load or an edge of a storage rack in which case the flexible chain extending from the body of the fork lift truck to the fork is permitted to slacken and result in an uncontrolled descent of the forks.
It is heretofore been suggested that parallel lines be provided between a three position control valve with a check valve in one line and reverse pressure relief valve in the other so that lowering will occur only when there is a predetermined pressure. However, such an arrangement has a disadvantage of increasing the pressure drop to and from the cylinder.
Accordingly among the objectives of the present invention are to provide a hydraulic lift system for fork lift trucks which obviates the problem of uncontrolled descent of the fork upon engagement of the fork with an object; which accurately is set at a pressure such that it is between that required to support the total fork assembly with no load on the fork and the pressure generated by the lift cylinder rod assembly; which does not have an undesirable pressure drop; and which does not have increase leak paths which would increase cylinder fork leakage.
In accordance with the invention a hydraulic lift valve system for fork lift trucks comprises a three way valve including a spool in a bore having a first neutral position, a second lift position and a third lower position. In the second lift position, the spool functions to apply flow of oil from a source to a cylinder for lifting the fork of the truck only after the pressure exceeds load pressure plus a predetermined value as established by a poppet valve. Upon shifting to the third position the spool functions to permit exhaust of the fluid in the cylinder and therefore lowering the fork only after a predetermined pressure is established in a pilot valve which permits the poppet valve to open, thereby insuring that the fork of the lift truck is not lowered unless there is a predetermined load on the fork. By this arrangement, the problem of uncontrolled descent of the fork upon engagement of the fork with an object is obviated.